How to handle double login in postman

I have an api to authenticate to the Igloo. I am using service account along with API key and password.
When i login on browser using my service account creds, i get first login page where i enter username and password> click ok. Then it redirects to something else and again i get similar logon box. When i enter same creds, it logs me in successfully.

Now when i run api in postman using POST method to generate session id, i get Authentication error( Bad password or something).

Why would that be.